The present invention generally relates to semiconductor devices and methods of producing the same, and more particularly to a semiconductor device having pin photodiodes and a method of producing such a semiconductor device.
When assembling or adjusting an optical communication system, it is becoming necessary to directly monitor an optical output of a semiconductor laser using an ultra high-speed semiconductor light receiving element which has a response speed of 30 GHz or greater because of the high transmission speed of optical signals used.
Generally, in order to increase the response speed of a pin photodiode, the diameter of the pin junction is reduced and the thickness of an i-type photoabsorption semiconductor layer is reduced. When such a measure is taken and the response speed is 30 GHz or greater, the diameter of the pin junction becomes 20 .mu.m or less and the thickness of the i-type photoabsorption semiconductor layer becomes 1 .mu.m or less.
As described above, it is difficult to produce a pin photodiode having a small pin junction diameter and a thin i-type photoabsorption semiconductor layer. But it is also extremely difficult to obtain an electrical signal which is obtained by a photoelectric conversion from such a small pin photodiode without signal deterioration for the following reasons. That is, the operating part of the pin photodiode itself can be made considerably small by using the present techniques for producing semiconductor devices. However, there is a limit to reducing the length and overall size of the wire bonding part which is used to obtain the electrical signal. As a result, there is also a limit to reducing the electrostatic capacitance and inductance.
On the other hand, for a pin photodiode having a pin junction diameter of 15 .mu.m and a photoabsorption layer with a thickness of 1.4 .mu.m and subjected to an ultra high speed optical signal of 20 GHz or greater, it is possible to realize a capacitance of 30 to 40 PF. But such a small photodiode easily breaks when subjected to by a surge voltage or the like.
In order to overcome this problem and realize a high-speed pin photodiode having a high reliability, there is a proposed pin photodiode which is provided with a chip capacitor. The chip capacitor is provided adjacent to the pin photodiode chip and acts as a bypass capacitor for bypassing the surge voltage. However, no matter how close the chip capacitor is provided adjacent to the pin photodiode chip, there is a limit depending on the size of the chip capacitor or the like, and this limit prevents complete elimination of the above described problem. In other words, it is difficult to positively prevent breakdown of the pin photodiode by quickly responding to even an impulse external noise.
The pin photodiode has an i-type region sandwiched between a p-type region and an n-type region. An avalanche photodiode (APD) has an i-type region in which an avalanche breakdown occurs. According to these light receiving elements, the i-type region functions as a photoabsorption layer which absorbs light, and the material used for the photoabsorption layer changes depending on the wavelength of the light used for the optical communication. A description will now be made of the conventional light receiving elements which are designed for optical communication using a light having a wavelength of approximately 1 .mu.m such as 1.3 .mu.m and 1.6 .mu.m.
FIG. 1A shows a conventional pin photodiode. In FIG. 1A, an n.sup.+ -type InP layer 552 is formed on a semiinsulative InP substrate 551. An undoped InGaAs layer 553 which forms an i-type region for absorbing light is formed on the n.sup.+ -type InP layer 552. An n.sup.- -type InP region 554 is formed on the InGaAs layer 553. A p.sup.+ -type region 555 is formed within the n.sup.- -type InP layer 554 by diffusing Zn, so as to form a pin structure. For example, the n.sup.+ -type InP layer 552 has a thickness of 2 .mu.m, the InGaAs layer 553 has a thickness of 1.7 .mu.m, and the n.sup.- -type InP layer 554 has a thickness of approximately 1 .mu.m which is doped with Si with an impurity concentration of 1.times.10.sup.15 cm.sup.-3.
The n.sup.- -type InP layer 554 and the InGaAs layer 553 are selectively etched to expose the surface of the n.sup.+ -type InP layer 552. An n side electrode 557 which is made of AuGe/Au, for example, is formed on the n.sup.+ -type InP layer 552. On the other hand, a p side electrode 558 which is made of AuZn/Au, for example, is formed on the p.sup.+ -type region 555. Because both sides of the InGaAs layer 553 which becomes the photoabsorption layer are formed by the InP having a wide band gap, the incident light can be received from the top surface to the bottom surface of the structure. When receiving the incident light from the top surface of the structure, an opening is formed at a central part 559 of the p side electrode 558.
When a predetermined reverse bias voltage is applied across the electrodes 557 and 558 of the pin photodiode shown in FIG. 1A, the InGaAs layer 553 essentially becomes a depletion layer. The electric field within the depletion layer accelerates the electron-hole pairs which are generated by the incident light, thereby causing the holes to be collected at the p side electrode 558 and the electrons to be collected at the n side electrode 557.
FIG. 1B shows a conventional lateral pin photodiode. In FIG. 1B, an undoped InP layer 661 which acts as a buffer layer is formed on the semiinsulative InP substrate 551. An undoped InGaAs layer 662 which acts as a photoabsorption layer is formed on the InP layer 661. An n.sup.- -type InP layer 663 is formed on the InGaAs layer 662. Zn is selectively diffused within the n.sup.- -type InP layer 663 so as to form a p-type region 664.
The InP layer 661 and the InGaAs layer 662 are made of a substantially intrinsic semiconductor having an impurity concentration of 1.times.10.sup.14 cm.sup.-3 or less. For example, the InP layer 661 and the InGaAs layer 662 have thicknesses of approximately 1 .mu.m and 1.7 .mu.m, respectively. For example, the n.sup.- -type InP layer 663 has a thickness of approximately 0.3 .mu.m and an impurity concentration of approximately 1.times.10.sup.-3. A p side electrode 668 which is made of AuZn/Au is formed on the p-type region 664, and an n side electrode 667 which is made of AuGe/Au is formed on the n.sup.- -type InP layer 663.
When a predetermined reverse bias voltage is applied across the electrodes 667 and 668 of the lateral pin photodiode shown in FIG. 1B, the pin junction in the periphery of the i-type region 664 becomes reverse biased and a depletion layer 665 spreads as indicated by a dotted line. When the incident light reaches the region of this depletion layer 665 and the electron-hole pairs are generated, the holes are collected at the p side electrode 668 and the electrons are collected at the n side electrode 667.
FIG. 1C shows a conventional metal-semiconductor-metal (MSM) photodiode. In FIG. 1C, the InP layer 661 which acts as the buffer layer is formed on the semiinsulative InP substrate 551 and the InGaAs layer 662 which acts as the photoabsorption layer is formed on the InP layer 661, similarly to the lateral pin photodiode shown in FIG. 1B. An undoped InAlAs layer 771 having a thickness of approximately 0.1 .mu.m is formed on the InGaAs layer 662 for forming a Schottky contact. Schottky electrodes 772 and 773 which are made of Al or the like are formed directly on the InAlAs layer 771. The electrodes 772 and 773 have comb shapes which intermesh with each other, as would be seen in a plan view. When a negative voltage is applied to the electrode 772 and a positive voltage is applied to the electrode 773 as shown, an electric field indicated by arrows is generated from the electrode 773 towards the electrode 772, penetrating the InGaAs layer (photoabsorption layer) 662. When the incident light reaches the InGaAs layer 662 and the electron-hole pairs are generated by the absorption of light, the electrons and holes are accelerated by the electric field and are respectively collected at the electrodes 773 and 772.
According to the pin photodiode shown in FIG. 1A, it is difficult to form the p and n side electrodes 558 and 557 on the same plane and it is inevitable to employ a mesa structure. As a result, it is difficult to integrate the pin photodiode together with other electronic elements.
Furthermore, in the case of the pin photodiode shown in FIG. 1A, it is necessary to use one mask for forming the n side electrode 557 and another mask for forming the p side electrode 558. For this reason, it is necessary to align the masks with a high accuracy particularly as the size of the photodiode becomes smaller, and a poor alignment of the masks would result in a poor performance of the photodiode. In addition, two different electrode materials are used to form the electrodes 557 and 558 using the two different masks, thereby making the production process complex.
In the case of the lateral pin photodiode shown in FIG. 1B, it is difficult to form a depletion layer under one electrode, that is, the n side electrode 667 in the case of the structure shown in FIG. 1B. For this reason, when the light is absorbed in the region where the depletion layer is not developed and the electric field is weak, the carriers cannot move at a high speed and the response speed of the lateral pin photodiode is poor.
According to the MSM photodiode shown in FIG. 1C, a barrier is generated at a hetero interface between the InAlAs layer 771 which forms the Schottky barrier and the InGaAs layer 662 which forms the photoabsorption layer, thereby forming a carrier trap. Consequently, it is difficult to obtain a sufficiently high response speed unless the composition of the InAlAs layer 771 is controlled so that the composition at the interface between the InAlAs layer 771 and the InGaAs layer 662 is close to InGaAs. In addition, a region is generated in which the electric field does not readily develop, immediately under the electrode. When the carriers are generated in such a region, the response speed of the MSM photodiode becomes poor.